1 World War II began in 1939. The United States did not join the war at first. Things changed in 1941. On December 7, Japan attacked Pearl Harbor. Pearl Harbor is in Hawaii. They attacked a U.S. Naval base. Over 2,000 Americans were killed. Many ships sank. The United States joined the war. They joined one day after the attack. By declaring war on Japan, the U.S. entered World War II.2 There is a memorial at this site today. It is in Hawaii. It is called the USS Arizona National Memorial. It is named for a battleship. The ship was sunk in the attack. The memorial is a bridge over the shipwreck. It honors the Americans who died. Tour boats take visitors over the wreckage. Sometimes, survivors of the attack are at the memorial. They tell their stories.